In the manufacture of many commercial products, a liquid composition is applied as a coating to a receptor substrate. In many such applications, as in the manufacture of imaging films and papers, the requirements for a real uniformity of coated thickness are highly demanding.
Known coating apparatus typically includes a backing roller around which a continuous web to be coated is wrapped and conveyed at a predetermined conveyance speed. A liquid composition is continuously delivered to and reshaped by an applicator, generally known as a hopper, from a jet flow at the applicator inlet into a broad ribbon of substantially uniform thickness at the applicator outlet from which it is dispensed onto the moving web. Typically, such an applicator is positioned either immediately adjacent to the moving web at a distance of typically less than 1 mm, a transverse, dynamic bead of composition being formed therebetween (bead coating), or above the web at a distance of typically several cm, the composition being allowed to fall as a curtain under gravity into continuous contact with the moving web (curtain coating). A liquid composition may be a single layer or a composite layer consisting of a plurality of coating compositions.
The moving web carries with it a boundary layer of air on the front side (the side to be coated) and the back side (the side facing the backing roller).
To prevent upsets in the coating and resulting coated thickness nonuniformities, each boundary layer must be eliminated before or at the coating point, which elimination becomes more difficult as coating speed is increased.
In all coating systems, there is an upper speed limit for coating at which the boundary layer of air carried on the front side is no longer squeezed out by the advancing composition at the coating point but rather becomes entrained under the composition, disrupting the uniform application thereof to the web and resulting in unacceptable coating uniformity.
It is well known that electrostatic charging of a web and/or coating apparatus can be useful in increasing this limit on coating speed, which process is referred to herein as electrostatic assist. For example, a dielectric web carrying a bound polar charge between opposite surfaces thereof can exhibit increased apparent "wettability" and a consequent increase in acceptable coating speed when conveyed around a grounded coating roller. Means for applying such a charge to a web ahead of the coating point are disclosed, for example, in European Patent No EP 390774; U.S. Pat. Nos. 4,835,004; 5,122,386; 5,295,039; and European Patent Application No. 0 530 752.
Apparatus and methods also have been proposed for maintaining a uniform charge on a web between the charging apparatus and the coating roller. See, for example, U.S. Pat. No. 4,835,004 and European Patent No. 0 530 752 which propose to prevent degradation of charge uniformity by imposing strict environmental controls around the web.
It is also known to apply electrostatic charge at the coating point by electrifying the surface of the coating roller itself. See, for example, U.S. Pat. Nos. 3,335,026; 4,837,045; and 4,864,460.
All of these techniques can be useful in electrostatically assisting the coating of a composition to a web by providing an electrostatic field between the composition and the backing roller at the point of coating. Such an assist acts to cause the composition to be drawn more aggressively toward the backing roller and thus to more forcefully squeeze out the front side boundary layer of air, permitting thereby an increase in coating speed which can be economically beneficial.
As noted above, a moving web also carries a boundary layer of air on its back side or surface as does the backing roller surface prior to engagement with the web. For every conveyance system there exists a speed at which conveyance is limited by back surface air entrainment between the web and the conveying roller. If the surface of the coating roller is smooth and the moving web is conveyed around the roller, then an air film will arise between the web and roller, creating an air bearing between the two surfaces. This air film thickness (h) is a function of several parameters: 1) coating roller radius (R), 2) dynamic air viscosity (.mu.), 3) web speed (U.sub.w), 4) roller speed (U.sub.R), and 5) web tension per unit width (T) and is given by the following equation: ##EQU1##
[Knox & Sweeney, IECP J., V. 10, 1972].
For a given air viscosity and web tension, the air film thickness will increase with increasing web/roller speed and/or roller diameter. This increase in air film thickness results in decreased contact between the web and roller, with a concomitant loss in traction. If the speed is increased to the point that the air film thickness is of the same order, or larger than, the roughness of either the smooth roller surface or the surface of the web facing the roller, then traction will be lost completely, resulting in slippage of the roller against the web. This loss of traction can result in problems such as cinches, scratches, tension and speed variations. In addition to loss of traction, the entrained air film thickness between the web and roller will result in reduced electric fields in the air gap between the web and the coating liquid, resulting in a significant reduction in the electrostatic assist and an associated occurrence of air entrainment.
It is known to provide means to remove or exhaust the boundary layers of air being carried on the back surface of a web and the surface of a roller when the two come into contact, increasing thereby the tractional contact of the web with the roller. Such means may include, for example, a pressure-loaded nip roller urged toward the conveying roller, the web passing therebetween. However, use of a nip roller may not be particularly desirable, as it adds mechanical complexity to the apparatus, and a face-side nip roller can mar the surface of the web to be coated and can cause electrostatic disturbance of either or both of the web surfaces, resulting in coating non-uniformities.
Such means may also include a relief pattern formed in the surface of the conveying roller into which the back-side boundary layer air may be exhausted from the web and escape. See U.S. Pat. No. 3,405,855 issued Oct. 15, 1968 to Daly et al., for example. In this patent, Daly et al. teach the use of a roller having peripheral venting grooves and supporting land areas to vent air carried by the underside of the traveling web. Another example is provided by U.S. Pat. No. 4,426,757 issued Jan. 24, 1984 to Hourticolon, et al. In this patent, Hourticolon, et al. teach the manufacture and use of a roller having a surface relief consisting of a "finely branched network of compression chambers", allowing the entrained air to be compressed into pockets rather than reducing the web traction. Both of these patents deal with purely conveyance roller issues and neither patent addresses the issue of electrostatic assist with such a roller surface pattern. It is not obvious that these roller surface patterns would perform well during an electrostatically assisted coating process, because these relief patterns will produce electrostatic field variations at the liquid-air interface of the coating composition. As shown in this invention, the resulting variations in the electrostatic force felt by the coating fluid can vary by more than a factor of ten. This local reduction in electrostatic force over the relieved surfaces will allow air entrainment at the front side (between the coating fluid and the web) to occur at lower speeds compared to those portions of the web with intimate contact between the roller surface and the back side of the web. Therefore, at intermediate speeds, it has been observed to obtain good coating over the portions of the web in intimate contact with the backing roller and air entrainment between the web and the coating solution over the relieved portions of the roll whereas at higher speeds, air entrainment is observed across the entire web.
As an example, using equation 1 and the parameter values given in the example by Hourticoulon (7 cm roller diameter, 380 m/min. web speed, and 15 kg/m web tension) yields an air film thickness of 10 .mu.m. This film thickness is smaller than the stated depths of the compression chambers of 30-80 .mu.m, and therefore a smooth roller under the same operating conditions would be expected to produce higher and more uniform electrostatic assist levels than a roller having the "finely branched network of compression chambers" described by Hourticoulon, et al. Similarly, the groove depths described by Daly, et al. are greater than 500 .mu.m, leading to the same unfavorable comparison with smooth rollers regarding electrostatic assist level and uniformity. Current state-of-the-art practice, therefore, is the use of a smooth backing roller when practicing electrostatically assisted coating. The use of a roller having a relieved surface pattern has been limited to use without electrostatic assist because of the above problems.
It has been found that the current practice of using a smooth backing roller while practicing electrostatically-assisted coating performs satisfactorily at relatively low web speeds, such as less than 75 meters/minute. However, at higher web speeds over backing rollers with diameters greater than or equal to about 10 cm, a thin air film is captured between the web and the smooth backing roller. This air film increases in thickness as web speed increases. Moreover, in contrast to the expectation from equation 1, it is observed on actual coating machines that this thickness is not constant but highly variable due to the numerous sources of variation present in the coating environment, and contact between the roller and the web may be intermittent, causing pockets of air to be entrained between the web and roller. For coating systems employing electrostatic assist, as the web is lifted off the backing roller, capacitance relationships among the backing roller, the web, and the coating applicator can change significantly, reducing locally the magnitude of electrostatic assist and thus permitting onset of air entrainment failure at the coating nip. These intermittent pockets of air will result in a reduction in electrostatic force that is larger than the reduction expected due to the uniform air film thickness predicted from equation 1.